1. Field of the Invention
The invention relates to heater rollers for use in a variety of machines, as well as methods of making such rollers, and circuits for detecting faults in such rollers.
2. Description of the Background Art
In recent years, the assignee of the present invention has introduced ceramic heater rollers as an alternative to steam-heated and induction-heated rollers used in the papermaking, printing and laminating industries and the paper, film, and foil converting industry. Such rollers are characterized by at least one insulating layer and by a heating layer of ceramic material that is heated by electrical current flowing through the layer. This type of roller operates as a resistance heating device, but this type of roller also acts as a capacitor, as a result of the insulating layer being positioned between two electrodes to form the capacitor.
A few milliamps of current may be available on the heater roller surface. It is desirable to have the voltage and current on the surface of the roller as close to zero as possible.
A second technical problem arises from cracks in the ceramic material which can be due to a variety of causes including: 1) operation at temperatures which are excessive, or at temperatures which vary greatly along the roller, or 2) sharp impacts caused by dropped objects or foreign objects in the processing line. Sometimes, these cracks are too small to be readily observed. Such cracks may further result in electrical faults occurring between components of the roller that would otherwise be insulated by the ceramic layers. Therefore, it would be desirable to have some method to monitor electrical faults in the roller which are caused by cracks in the ceramic.
The present invention is directed to improved constructions of ceramic heater rollers, as well as associated control equipment, and to improved methods of making such heater rollers.